Treating glass compositions

ABSTRACT

A process for treating a halide, e.g. fluoride, glass composition, characterized by contacting a melt of the composition with dry oxygen. Oxygen, simply on contact with a fluoride glass melt, converts transition metals, e.g. from Fe(II) to Fe(III), from one stable state to another and thereby reduces loss at transmission wavelengths in optical fiber, e.g. the loss at 2.7 μm attributable to Fe(II).

FIELD OF THE INVENTION

This invention relates to the treatment of glass compositions. Inparticular, it relates to the treatment of compositions which aresuitable for use in the production of optical fibres, in order that thefibres should have low loss.

BACKGROUND OF THE INVENTION

The distance over which a signal can be successfully transmitted throughan optical fibre is limited by two distinct types of loss which occur inglasses generally: losses arising from absorption, e.g. in electronictransitions or vibrational transitions; and losses due to Rayleighscattering (a phenomenon resulting from inhomogeneities). The lattertype of loss is a theoretically inevitable one, although it becomes farless significant at longer wavelengths. If a glass can be formulated sothat the absorption is very low at long wavelengths, very low loss(absorption plus Rayleigh scattering) can be achieved in communicationsat such wavelengths.

In germanosilicate and borosilicate glass fibres, the absorptionspectrum is such that they have minimum loss in the range of 0.8-1.2 μm.Such fibres are less suitable for long than for short distancecommunication, e.g. between computers. An absorption peak at about 950nm may be attributed to OH vibrational absorption, and various solutionshave been propoed to reduce loss at or around this wavelength, by thepassage of gases through the glass melt, such gases being CO/CO₂(GB-A-1507712), dry oxygen (GB-A-2033373), chlorides such as SiCl₄(JP-A-56-149332) and fluorine-containing gases, e.g. C₃ F₈ and F₂, asdrying agents (EP-A-0103441).

Loss may also be attributable to impurities in the glass constituentsand therefore in the glass. One class of impurities is the transitionmetal elements, but the need to reduce the loss caused by such compoundsis dependent on the relationship between the wavelength at which theyabsorb and the transmission wavelengths; if these wavelengths aresufficiently different, the impurities can be tolerated.

GB-A-1507712 (see above) discloses that the effect of Fe and Cuimpurities in oxide glasses may be reduced, not by removing thecompounds as such but by changing the oxidation states of the metals, toFe(III) and Cu(I), respectively. As₂ O₃ and other redox buffering agentsare proposed for use with the reducing gas CO.

By these various means, it has been sought, in theory or practice, toreduce the absorption losses inherent in optical fibres. A complementaryor alternative procedure has been to reduce Rayleigh scattering byformulating glasses which can be operated at longer wavelengths. Silicaglasses have loss minima at 1.3 and 1.55 μm and are already beingmanufactured and used for long distance communication. More recently,fluoride glasses have been prepared which, it is suggested, might beoperated in the 2 to 10 μm range. It seems more likely that, in theimmediate future, the operational range for such fibres will be of theorder of 3 μm. As operational wavelengths increase, there iscorresponding difficulty in formulating suitable lasers.

Preferred multi-component fluoride glass compositions are ZrF₄ -based.Examples of components of such compositions are, in addition to ZrF₄ (orHfF₄), BaF₂, LaF₃, GdF₃, AlF₃ and NaF or LiF. Particular compositions,and a general background to the formulation of non-silica-basedinfra-red fibres, are given by Miyashita et al., IEEE Journal of QuantumElectronics, QE-18, No. 10 (October 1982).

Some multi-component halide glasses are exemplified in Table 1 of thearticle by Miyashita et al. Suitable compositions can be determined fromphase diagrams of the type given for a ZrF₄ -BaF₂ -GdF₃ composition inFIG. 3 of the article by Miyashita et al.

Robinson et al, Mat. Res. Bull. 15 (1980) 735-742, report the use ofCCl₄ as a drying agent in fluoride glasses. Halogen andhalogen-containing compounds are generally known for use in reactiveatmosphere processing.

Tran et al, Sixth Topical Meeting on Optical Fiber Communication (Feb.28-Mar. 2, 1983), New Orleans, Digest of Technical Papers, page 7,disclose reactive atmosphere processing of ZrF₄ -based glasses usingSF₆, HF, CCl₄, CF₄ and NH₄ HF₂, to effect OH removal.

Bansal et al, J. A. Ceram. Soc. 66(4) (1983) 233, disclose reactiveatmosphere processing of ZrF₄ -based glasses under Cl₂, in a study ofcrystallisation kinetics.

Lecoq et al, Verres et Refractaires 34(3) (1980) 333-342, describe therole of Al as a stabiliser in ZrF₄ -based glasses. The glass compositionwas made by melting the constituents under ambient air, "which causespartial hydrolysis of the material at the moment of casting, but withoutpreventing the production of glass if the amount of the sample issufficient (>5 g)". Air, of course, includes water and dust; both canhave undesirable effects, by reaction or physical incorporation, on theproperties of, say, optical fibre.

Almeida et al, J. Non-Cryst. Solids 56 (1983)63-68, disclose that apronounced effect on the IR absorption edge of bulk ZrF₄ -based glassesis observed if oxide impurities are present, and that oxygen atoms tendto occupy bridging positions in the ZrF₄ chain-like glass skeleton.

Fe(II) and Cr(III), for example, have been observed in fluoride glasscompositions. It is reasonable to infer that these are the onlyoxidation states in which these transition metals can exist, stably, inan fluoride glass matrix.

It is well known that oxygen must be excluded when preparing fluorides.It appears also that oxides and oxygen atoms should not be introducedinto fluoride glass compositions. However, there is as yet nosatisfactory solution to the problem of reducing absorption losses insuch compositions, without going to economically unacceptable lengths topurify the constituent materials.

SUMMARY OF THE INVENTION

According to the present invention, a process for treating a halideglass composition to improve infrared transmission characteristicscomprises contacting a melt of the composition with dry oxygen. The useof oxygen is convenient and practical, and avoids problems associatedwith handling other oxidising agents such as F₂, Cl₂ andperfluorocarbons. The process can be conducted in simple apparatus ofthe type illustrated schematically in FIG. 1 of the accompanyingdrawings, which is described in more detail below.

The present invention is based on the realisation that the raw materialswhich are used to prepare a halide glass contain carbon and transitionmetal impurities which can have a deleterious effect on the absorptionspectrum of the glass, and on the attenuation of optical fibre preparedfrom the glass. It has now been appreciated that, at the likelyoperational wavelength for fluoride glass fibres, Fe(II) contributes toloss, although Fe(III) does not. This is similar to the problemsencountered in oxide glasses, as discussed above, but a difference liesin the fact that losses due to Cu(II) absorption can be substantiallyignored at higher wavelengths than are used for transmission in oxideglass fibres. It has been discovered that fluoride glasses can containtransmission metal impurities in different oxidation states, stablywithin the matrix; this had not been predicted, and was unexpected.

The present invention is based also on the surprising discovery that theuse of oxygen does not necessarily cause the production of oxides orother compounds in the glass, which might absorb at or near suitableoperating wavelengths. This can now be explained, with hindsight, by therelatively high thermodynamic stability of fluorides with respect tooxides. It has also been found that reactive atmosphere processing isunnecessary for the removal of OH from fluoride glasses.

In the invention, the glass may be any halide, e.g. mixed halide, glasscomposition. A fluoride glass composition is preferred.

Oxygen will usually be used, in the invention, in admixture with aninert gas diluent. Air can be used, if well dried, e.g. by usingmolecular sieves or other filters. Thus, for example, an inert gas suchas N₂ may be used, the gas mixture usually containing at least 1 andgenerally at least 5, preferably 5 to 50, e.g. 10, % v/v O₂.

If appropriate, O₂ can be generated in situ. For example, a source of N₂O may be used; O₂ and inert gases only are then in contact with theglass melt, under the reaction conditions. As a further example, asuitable liquid which releases O₂ on heating can be volatilised and thevapour caused to pass over the glass melt.

It is of course desirable that the oxygen should not be used in thepresence of components which can themselves be, or react to introduce,undesirable components in the composition. Moisture must be absent, inorder to avoid the formation of oxides. Other components of ambient air,such as dust, should also be avoided. The glass may be melted in acrucible which should be inert.

It is conventional to contact an inert gas with a glass melt, and such aprocedure can simply be modified in accordance with the invention. Theoxygen may be brought into contact with the glass composition in aconventional schedule of the type used for bubbling gases throughborosilicate glasses, e.g. as described in EP-A-0103441, althoughbubbling is not necessary in this case. Oxygen can simply be flushedover the melt, in a sealed chamber, to give satisfactory results.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows apparatus upon which the inventor's process can bepracticed.

FIGS. 2 and 3 show absorption spectra for the glasses discussed inExamples 4 and 5, respectively.

DETAILED DESCRIPTION

FIG. 1 shows a suitable sealed chamber. Within the chamber, there is acrucible 1 having a lid 2 and containing a glass melt 3. The crucible 1is housed within a linear 4 having a lid 5 through which gas can entervia an inlet 6 and exit via an outlet 7. The crucible 1 and the liner 4are positioned inside a furnace 8. The crucible may be of Pt/Au, theliner and its lid of silica.

The schedule of oxygen/melt contact will in fact usually comprise aperiod during which an inert gas/oxygen gas mixture is contacted withthe melt, followed by an inert gas phase which allows cooling. Theperiod of oxidation may be from 30 min to 5 hours. The temperatureduring oxidation may be from 600 to 1200 C.

The desired effect of the process of the invention can be monitored byobservation of the absorption characteristics of the bulk glass and, inparticular, by monitoring the disappearance of Fe(II) absorbence in the2-3 μm wavelength range, or the appearance of absorbence owing toFe(III). The reaction should go substantially to completion, i.e. sothat there is substantially no Fe(II) absorbence, a characteristic whichcan be determined by relative experimentation at least.

Compositions treated by the invention can be formulated into bulkglasses or preforms for subsequent drawing into optical fibres, e.g. formonomode transmission. It is preferred that both core and claddingglasses of an optical fibre are treated by the process of the invention.Fast quenching may be necessary. Chemical vapour deposition may be used.

Glass compositions treated by the invention can also be used for opticalcomponents in general, e.g. windows and test apparatus, and whereverthere is a need for long wavelength transmission.

Fluoride (and other halide) glass fibres can be produced by the methodsdescribed by Miyashita et al., supra. These methods can be used orsuitably modified for the production of fibres having a core and acladding, e.g. from a double-crucible or by casting; see also Mitachi etal., Electron Lett. 18 (Feb. 1982) 170-171. The core diameter may be 5to 200 μm, and the cladding's outer diameter is usually at least 25 μmmore, e.g. 100 to 300 μm. Optical fibres of the invention may have lowloss, e.g. less than 100 dB/km (at 2.8 μm).

The following Examples illustrate the invention.

EXAMPLE 1

Two glass compositions (about 20 g and 30 g, respectively) were preparedfrom ZrF₄, BaF₂, LaF₃, AlF₃, NaF, PbF₂ (in the first case) and NH₄ HF₂.The amounts of these components in the first (core glass) compositionwere 11.71, 4.65, 1.41, 0.37, 1.03, 0.83 and 0.5 g, respectively; in thesecond (cladding glass) composition, these amounts were 18.66, 7.39,1.68, 0.53, 1.76, 0 and 0.5 g, respectively.

The compositions were heated to 400 C in separate, inert crucibles withlids, under a nitrogen gas flow at a rate of 3 l/min. After 45 minutes,the temperature of the oven was raised to 900 C. After 60 minutes,oxygen was also introduced, at a rate of 0.2 l/min, and the combinednitrogen/oxygen flow was maintained for 2 hours. The oxygen flow wasthen turned off and the temperature was reduced to 670 C. After afurther hour, the melts were cast to form a preform, by centrifugallycasting the cladding glass into a tube and then casting the core glassinto that tube, followed by cooling and annealing.

The preform was drawn into fibre having an external diameter of 180 μm.The loss characteristics of the fibre were observed. An attenuationmaximum corresponding to about 1 ppm Cu(II) was observed at about 1 μm,and an attenuation minimum, apparently substantially free of loss owingto Fe(II), at 2.6-2.7 μm.

EXAMPLE 2

The procedure of Example 1 was followed, except that a temperature of850, rather than 900, C was used and the O₂ flow rate was increased to0.3 l/min. The product was again satisfactory, with little or no lossattributable to Fe(II).

EXAMPLE 3

A series of tests was conducted to illustrate the utility of theinvention. A glass composition was made up from anhydrous fluoridepowders which had been stored under dry nitrogen. The composition was51.5% ZrF₄, 19.5% BaF₂, 5.3% LaF₃, 3.2% AlF₃, 18.0% NaF and 2.5% PbF₂(percentages by weight). The powders were weighed and mixed into plasticcontainers in batches of 30 g. In general, 0.1 wt % metal fluoride wasadded to each batch, using the materials shown in the Table, below. Inaddition, 0.5 g NH₄ HF₂ was also added, to convert any residual oxidesinto fluorides.

In testing, a batch was transferred to a Pt/Au crucible fitted with alid and housed within a sealed silica liner, as shown in theaccompanying drawing. Dry nitrogen was flushed through the liner. Atypical melt schedule is shown below:

    ______________________________________                                        TIME INTO                                                                     RUN (h)  TEMPERATURE (C.)  GAS                                                ______________________________________                                        0        400               N.sub.2 (3 l/min)                                  0.75     850                                                                  1        850               O.sub.2 (0.3 l/min)                                3        670                                                                  4        Quench            N.sub.2                                            ______________________________________                                    

All glasses were melted under both oxidising and reducing conditions.Oxidising conditions were provided by introducing 0.3 l/min of O₂ intothe N₂ gas flow, 1 hr into the run, and maintaining this until the end.The pO₂ in this case was 0.091 atmospheres. Reducing conditions wereobtained by melting in a Pt/Au crucible under dry nitrogen supplied froma large reservoir of liquid nitrogen (-196 C). Since the meltingtemperature was kept constant and the N₂ supply was from a continuoussource with a quoted oxygen content of less than 10 ppm, the oxygenfugacity is believed to have been fairly constant between runs.

After 4 hr (total melting time), the liner was removed from the furnaceso that the melt could be quenched whilst still under an atmosphere ofdry nitrogen. After a quench time of 5 min, the glass had reached Tg,and the crucible and glass sample were transferred to an annealing ovenat 240 C. The sample was cooled overnight to room temperature, beforebeing removed from the crucible and cut and polished in preparation foroptical measurements, to thicknesses in the range of 6 to 24 mm.

Absorption spectra were measured over a wavelength range of 0.2 to 2.85μm. In general, the colours observed in the absorption spectra weresimilar to those seen in low alkali oxide glasses where the transitionmetals take up octahedral symmetry, apart from Fe and Cu where theweaker ligand field associated with fluoride has removed the normalcolouration and shifted the absorption into the infra-red.

In order to confirm that the appropriate dopants had been dissolved intothe melt, dopant concentrations were independently measured on eachsample using electron microprobe techniques. The work was done with aCambridge Instruments S180 scanning electron microscope fitted with aMicrospec wavelength dispersive X-ray spectrometer. Small circularblocks, about 6 mm in diameter, were cut from each glass sample andmounted in a large block. The top surface was then polished flat, coatedwith gold and mounted for electron microprobe work.

The results from the microprobe work are given in the Table, andcompared with the expected dopant concentrations. The large error of±20% on these measurements arises from the lack of suitable standards.Nevertheless, within experimental error, the respective values agreewell. The Table shows that, in several cases, the detectedconcentrations were considerably lower than those expected and, inparticular, Ti and V were low in the oxidised samples; Ti, Co, Ni and Cuwere low for the reduced cases.

Since TiF₄ and VF₅ are very much more volatile than the other glasscomponents, it is likely that these evaporated out of the melt. Underreducing conditions, it is likely that Ni and Cu partially precipitatedout of the melt as the reduced metal (Cu⁰ etc.). In fact, for the caseof Cu in particular, it could be seen that Cu metal was left on thewalls of the Pt/Au crucible after melting. In all cases, some traces ofdopant were still dissolved in the glass.

                  TABLE                                                           ______________________________________                                        (MICROPROBE RESULTS)                                                                           Detected Level (± 20%)                                    Dopant  Dopant Level   Oxidised Reduced                                       ______________________________________                                        TiF.sub.4                                                                             390             20      --                                                    1000           --       150                                           VF.sub.4                                                                              400            --       300                                                   1000            60      --                                            CrF.sub.3                                                                             480            400      --                                                    1000           --       1100                                          MnF.sub.2                                                                             590            --       450                                                   1000           800      --                                            FeF.sub.2                                                                             600            500      550                                           FeF.sub.3                                                                             500            --       500                                           CoF.sub.2                                                                             600            550      350                                           NiF.sub.2                                                                             600            400      100                                           CuF.sub.2                                                                             620            500      100                                           ______________________________________                                    

EXAMPLE 4

Bulk glasses were made up from anhydrous fluorides and doped with Fe andCu. The host glass was a Zr-Ba-La-Al-Na-Pb fluoride composition whichwas used as the core glass in IR fibres. Oxidised and reduced glasseswere made up by melting under a stream of N₂, either alone (reducing) ortogether with O₂.

The absorption spectra for these glasses are shown in FIG. 2 as afunction of the extinction coefficient (abscissa) in dB/km/ppm withrespect to wavelength (ordinate) in μm. Under reducing conditions, alarge peak is apparent, centred at 1.12 μm with a side shoulder at 1.75μm. This has been attributed to Fe²⁺ in distorted octahedral symmetry;see Ohishi et al, Phys. & Chem. Glasses 24 (1983) 135-140. Underoxidising conditions, Fe absorption is removed as Fe²⁺ is oxidised toFe³⁺. An alternative peak is developed, centred at 0.97 μm. This can beattributed to Cu²⁺ in tetragonal symmetry. Although there are twoconflicting effects, it should be noted that the absorption due to Cu²⁺tails off more rapidly beyond 1.5 μm than that due to Fe²⁺. Since Feusually occurs at a higher leel of impurity than Cu, it can be concludedthat, in order to minimise overall absorption losses, oxidisingatmosphere should be used.

EXAMPLE 5

Infra-red fibres were fabricated from both oxidised and reduced glassesin order to determine the effects on the transmission losses. The coreglass was of the same composition as that described in Example 4, andthe cladding glass was similar but with a lower refractive index.Reducing conditions were again obtained from a flow of dry N₂ ; O₂ wasintroduced to give oxidising conditions. The glasses were converted intopreforms using rotational casting as described by Tran et al, Electron.Let. 18 (1982) 657-658, and were drawn into fibre using conventionaltechniques. The fibres were 170 μm in diameter with a core of 90 μm andloss measurements were made on 60 m of fibre cut from a 300 m length.

FIG. 3 shows absorption curves a and b, for fibres made respectivelyunder reducing and oxidising conditions, as a function of the extinctioncoefficient (abscissa) in dB/km with respect to wavelength (ordinate) inμm. The fibres have similar characteristics, with a minimum lossoccurring close to 2.7 μm, an OH absorption at 2.87 μm and an IR edgerising above 3.3 μm. The reduced fibre, however, clearly exhibits anFe²⁺ absorption corresponding to about 600 ppb of Fe (assuming all Feexists as Fe²⁺). The oxidised fibre has little Fe²⁺ absorption, but nowexhibits a peak centred at 0.97 μm corresponding to about 100 ppb ofCu²⁺. By removing the Fe²⁺ absorption and reducing scattering losses,the total loss in this fibre has been reduced to 21 dB/km at 2.7 μm.

More detailed studies of the loss of fibre (b) at shorter wavelengthsallow estimates to be made of the impurity levels of several transitionmetals and rare earths.

Fe is found to be the dominant impurity (Fe<600 ppb, Cu<100 ppb, allothers <80 ppb) and, for this reason, it is likely that oxidised glasseswill give lower absorption losses. The absoprtion loss of fibre (b) hasbeen estimated to be of the order of 2 dB/km at 2.5 μm and this figurehas been corroborated by measurements at shorter wavelengths usingcalorimetric absorption; see White et al, Opto-Electron. 5 (1973)323-324.

What is claimed is:
 1. A method of improving the infrared transmissioncharacteristics at 2.7 microns and of preparing a halide glasscomposition containing iron in the Fe(II) state, the method comprisingoxidizing impurities therein under dry conditions so as to substantiallyimprove infrared transmission characteristics at 2.7 microns, such thatsubstantially all said iron is converted to the Fe(III) state, whereinsaid oxidation is carried out by contacting said melt with oxygenwithout exposing the melt to ambient air, whereby a halide glasscomposition is prepared having substantially improved infraredtransmission characteristics and having an attenuation of less thanabout 100 dB/km at wavelengths on the order of 2.7 microns.
 2. A methodaccording to claim 1, in which said dry oxygen is flushed over the melt,in a sealed chamber.
 3. A method according to claim 2, in which said dryoxygen is in admixture with an inert gas.
 4. A method according to claim1, in which the composition is a fluoride glass composition.
 5. A methodof improving the infrared transmission characteristics at 2.7 micronsand of preparing a halide glass composition, the method comprisingoxidizing impurities therein under dry conditions, so as tosubstantially improve infrared transmission characteristics at 2.7microns, wherein said oxidation is carried out by contacting at melt ofsaid composition with oxygen without exposing the melt to ambient air,whereby a halide glass composition is prepared having substantiallyimproved infrared transmission characteristics and having an attenuationof less than about 100 dB/km at wavelengths on the order of 2.7 microns.6. A method according to claim 5, in which said dry oxygen is flushedover the melt, in a sealed chamber.
 7. A method according to claim 6, inwhich said dry oxygen is in admixture with an inert gas.
 8. A processaccording to claim 5, in which the composition is a fluoride glasscomposition.
 9. A method of preparing a halide glass compositionsuitable for making low loss optical fiber so as to substantiallyimprove infrared transmission characteristics at about 2.7 microns, themethod comprising:(a) melting together the components of the compositionin contact with an inert gas; (b) oxidizing the impurities in thecomposition by contacting said melt with a dry mixture of oxygen and aninert gas; and (c) cooling the composition in contact with an inert gas,whereby a halide glass composition is prepared having substantiallyimproved transmission characteristics and having an attenuation of lessthan about 100 dB/km at wavelengths on the order of 2.7 microns.
 10. Themethod according to claim 9 wherein the dry mixture includesapproximately 15 liters of inert gas for every liter of oxygen.
 11. Themethod according to claim 9 wherein the inert gas is nitrogen.
 12. Amethod of preparing a halide glass composition suitable for making lowloss optical fiber so as to substantially improve infrared transmissioncharacteristics at about 2.7 microns, the method comprising:(a) meltingtogether the components of the composition in contact with an inert gas;(b) oxidizing the impurities in the composition by contacting said meltwith oxygen in the absence of components which can react with the glasscomposition; and (c) cooling the composition in contact with an inertgas, whereby a halide glass composition is prepared having substantiallyimproved transmission characteristics and having an attenuation of lessthan about 100 dB/km at wavelengths on the order of 2.7 microns.
 13. Amethod of improving the infrared transmission characteristics at 2.7microns and of preparing a fluoride glass composition containing iron inthe Fe(II) state, the method comprising oxidizing impurities thereinunder dry conditions so as to substantially improve infraredtransmission characteristics at 2.7 microns, such that substantially allsaid iron is converted to the Fe(III) state, wherein said oxidation iscarried out by contacting said melt with oxygen without exposing themelt to ambient air, whereby a fluoride glass composition is preparedhaving substantially improved transmission characteristics and having anattenuation of less than about 100 dB/km at wavelengths on the order of2.7 microns.
 14. A method according to claim 13, wherein said oxidizingstep includes the step of exposing the melt to a mixture of dry oxygenand an inert gas.
 15. A method of improving the infrared transmissioncharacteristics at 2.7 microns and of preparing a fluoride glasscomposition, the method comprising oxidizing impurities therein underdry conditions, so as to substantially improve infrared transmissioncharacteristics at 2.7 microns, wherein said oxidation is carried out bycontacting a melt of said composition with oxygen without exposing themelt to ambient air, whereby a fluoride glass composition is preparedhaving substantially improved transmission characteristics and having anattenuation of less than about 100 dB/km at wavelengths on the order of2.7 microns.
 16. A method according to claim 15 wherein said oxidizingstep includes the step of exposing the melt to a mixture of dry oxygenand an inert gas.
 17. A method of preparing a fluoride glass compositionsuitable for making low loss optical fiber so as to substantiallyimprove infrared transmission characteristics at 2.7 microns, the methodcomprising:(a) melting together the components of the composition incontact with an inert gas; (b) oxidizing the impurities in thecomposition by contacting said melt with a dry mixture of oxygen and aninert gas; and (c) cooling the composition in contact with an inert gas,whereby a fluoride glass composition is prepared having substantiallyimproved transmission characteristics and having an attenuation of lessthan about 100 dB/km at wavelengths on the order of 2.7 microns.
 18. Themethod according to claim 17 wherein the dry mixture includesapproximately 15 liters of inert gas for every liter of oxygen.
 19. Themethod according to claim 17 wherein the inert gas is nitrogen.
 20. Amethod of preparing a fluoride glass composition suitable for making lowloss optical fiber so as to substantially improve infrared transmissioncharacteristics at 2.7 microns, the method comprising:(a) meltingtogether the component of the composition in contact with an inert gas;(b) oxidizing the impurities in the composition by contacting said meltwith oxygen in the absence of components which can react with the glasscomposition; and (c) cooling the composition in contact with an inertgas, whereby a fluoride glass composition is prepared havingsubstantially improved transmission characteristics and having anattenuation of less than about 100 dB/km at wavelengths on the order of2.7 microns.